sys/syscalls: drop outdated comment on _sbrk_r()
sys/syscalls: drop outdated comment on _sbrk_r()

miri64/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT